I believe the preference is to do some marshalling first and then do the IMC because you will have some idea what it is all bout having done some first. It is expensive for the TTMA to run the courses and would not want to have you train and then find out on the first occasion of actually marshalling that it is not for you. There are many aspects to marshalling and you will not be asked to do something you are not comfortable with. None of us can be 100% sure of how we will react first time to a major incident.